The microdevices manufactured by MEMS technology are playing key roles in many areas. For instance, micromechanical gyroscopes have enabled several important control systems in transportation and commercial applications. Other microdevices such as pressure sensors, accelerometers, actuators and resonators fabricated by MEMS technology are also used in many areas.
Some microdevices, such as micro gyroscopes and resonators contain a microstructure that needs to be maintained within a vacuum-sealed cavity. For these types of devices, there is a continuing need to improve the methods and techniques to hermetically seal the cavity to increase device lifetime. Pressure increases due to several sources can degrade device performance and reduce device lifetime for hermetically sealed microdevices.
One source that may cause pressure to increase in the cavity of a microdevice is inadequate sealing methods and techniques. For example, a microstructure has electrodes that need to be electrically connected to the outside environment in an appropriate way to meet further level packaging requirements, for instance, a surface mount capability. A suitable feedthrough design and hermetic seal method around the feedthroughs are needed to ensure a complete electrical connection and an appropriate cavity vacuum level over a device's lifetime.
For the wafer level hermetical seal, it has been known to electrically connect the electrodes of a microstructure through certain types of conductive vias formed in a wafer. This approach, however, has several disadvantages. For example, the formation of conductive vias in a wafer may result in mechanical damage to the microstructure mounted on the wafer. Additionally, vacuum degradation may occur over time due to micro cracks or other defects in the conductive via. This is especially true if the conductive via extends into the vacuum-sealed cavity that houses the microstructure. Moreover, the size of the microdevice increases with the number of vias required for connecting to the microstructure.
Another source of pressure increase in the cavity of a microdevice is from gas generation during the hermetic sealing process and outgassing from the package material, sealing material, and components within the cavity. With respect to outgassing problems, it has been known to maintain a sealed vacuum within a cavity by using getters to adsorb vapor and gas species. Two different kinds of getters currently used in devices are metallic getters and non-metallic getters. It has been known to use the metallic getters in package level vacuum sealing methods. A non-metallic getter formed from organic salts of silicon for an electron tube application is described in U.S. Pat. No. 4,771,214. Another non-metallic getter formed from deposited amorphous silicon or poly-silicon for flat panel display applications is described in U.S. Pat. No. 5,614,785.
Conventional procedures have been met with varying degrees of success. For instance, with metallic getters there is a serious reliability issue caused by getter particles falling down during fabrication process or after device experiencing vibration or shock due to poor mechanical strength and too large pore size of the used metallic getter. The presence of separated getter particles has been identified as a major failure mode for some micro gyroscopes sealed with porous metallic getters. Additionally, because metallic getters typically have large pore size, the required size of the getter is normally large. This size restriction and the getter fabrication process normally prohibit metallic getters from use in wafer level vacuum seals. Metallic getters are also cost prohibitive for some applications.
With relation to non-metallic getters, the mechanical properties of known amorphous or poly-crystalline silicon will change with deposition condition and are difficult to repeat. Known types of non-metallic getters are typically used in large sized cavities with large planar areas because of their limited thickness of only a couple of microns. Other types of getters are directed to adsorbing moisture within a cavity that is not perfectly hermetic.
It is, therefore, desirable to provide an improved microdevice having a microstructure residing in a hermetically sealed cavity of the microdevice and methods of making the same to overcome most, if not all, of the preceding problems.
While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.